The present invention generally relates to the analytical detection of nitro-containing compositions including nitrate and nitrite materials, and more particularly to the detection of such materials in water samples and on solid substrates using ultraviolet photolysis.
Current developments in chemical technology have created a corresponding need for rapid and accurate chemical detection methods. This need exists in a wide variety of technical fields. For example, excess nitrate and nitrite materials in ground and surface water can cause numerous problems. High levels of dissolved nitrate and/or nitrite compositions in ground and surface water are generally a consequence of large-scale inorganic fertilizer application. Improper or inadequate sewage treatment may also result in the contamination of water and soil materials with human/animal waste matter. The biological decomposition of such matter releases nitrate materials into the water and surrounding soil, thereby causing contamination problems. As a general rule, increased levels of nitrate and nitrite materials in ground and surface water occur through the leaching of dissolved inorganic nitrates and decomposition of organic nitrogen-containing compounds to form highly mobile nitrate ions (NO.sub.3.sup.-) and nitrite ions (NO.sub.2.sup.-). In aqueous environments, nitrate contamination dominates over nitrite contamination since nitrite ions are easily oxidized to form nitrate ions.
In both surface and ground water, it is important to accurately monitor dissolved nitrate and/or nitrite materials on a qualitative and quantitative basis. In the United States, the National Academy of Sciences (NAS) Committee on Water Quality Criteria recommends that the level of nitrate nitrogen in public water supplies not exceed 10.0 mg of nitrate nitrogen/liter of water and 1.0 mg of nitrite nitrogen/liter of water. These parameters are likewise endorsed by the United States Environmental Protection Agency (EPA) Office of Drinking Water. Nitrate and nitrite ion levels in drinking water which exceed the foregoing amounts can produce various health problems. For example, one problem of substantial concern involves an illness commonly known as "blue baby syndrome" experienced by infants which are bottle fed using water containing large quantities of nitrate ions. Because infants normally have a low level of digestive tract acidity, their digestive systems are capable of supporting the growth of bacteria which reduce ingested nitrates to nitrites. These nitrites thereafter enter the circulatory systems of the infants and ultimately cause a substantial reduction in blood oxygen-carrying capacity. Oxygen-deficient blood exhibits a characteristic blue color, thereby supporting the term "blue baby syndrome". The inadequate transport of oxygen in this manner can result in numerous health problems, including but not limited to arrested physical and mental development. Furthermore, studies have indicated that increased consumption of nitrate materials (including nitrate ions in water) can cause stomach cancer, as well as other illnesses.
Many attempts have been made in the past to quantitatively and qualitatively analyze nitrate and nitrite ion levels in water. For example, one commonly-used method involves ion chromatography in accordance with American Public Health Association (APHA) Method Number 4110. Ion chromatography involves the use of sophisticated chromatographic systems, and is capable of sequentially analyzing water samples for nitrate and nitrite levels. The applicable range of nitrate analysis using ion chromatography is about 0.03-3.4 mg of nitrate nitrogen/liter of water.
Another commonly-used nitrate analysis method involves cadmium reduction in accordance with APHA Method Number 4500-NO.sub.3.sup.- E. This method involves the reduction of nitrate ions to nitrite ions in the presence of cadmium. The resulting nitrite ions are then diazotized with sulfanilamide and subsequently coupled with N-(1-naphthyl)-ethylenediamine dihydrochloride to form a highly colored dye solution. The dye solution is thereafter calorimetrically analyzed, with the ultimate nitrate concentration being determined using a calibration curve. The detection range associated with this method is about 0.01-1.0 mg of nitrate nitrogen/liter of water.
An additional technique involves the use of an ultraviolet spectrophotometer system in accordance with APHA Method Number 4500-NO.sub.3.sup.- B. This method specifically uses an ultraviolet spectrophotometer in which ultraviolet light is applied to a liquid sample, followed by measurement of the ultraviolet absorbance characteristics of the sample. Such measurements are made without any dissociation or chemical alteration of the nitrate ions. In particular, this method measures nitrate concentrations using ultraviolet absorbance at about 220 nm, with an applicable detection range of about 0.02-11 mg of nitrate nitrogen/liter of water. However, ultraviolet spectrophotometry is not recommended for use with sample materials having substantial amounts of organic compositions therein. Such compositions may interfere with the spectrophotometer system. Other materials which may cause similar interference problems include but are not limited to various commercial surfactants and Cr.sup.+6 ions. It should likewise be noted that a variation of the foregoing method exists which involves the use of a fiber optic-based ultraviolet spectrophotometer system for continuous water monitoring as discussed in MacCraith, B. D., et al., "Fiber Optic Sensor for Nitrates in Water", Society of Photooptical Instrumentation Engineers--Chemical and Medical Sensors, 1510:195-203 (1991).
Notwithstanding the foregoing analytical methods and systems, a need exists for a method suitable for detecting nitrate/nitrite materials and other nitro-containing compositions which is rapid, efficient, and avoids the use of multi-step processes involving potentially dangerous chemical reagents (including but not limited to cadmium compounds and the like). Furthermore, a need exists for a detection method involving the foregoing materials which substantially avoids interference problems associated with organic materials, metal ions, and the like. The present invention satisfies this goal by providing a highly effective analytical process for detecting nitrate and/or nitrite ions in water (as well as dissolved non-ionic nitro-containing compounds) which is characterized by an absence of expensive and potentially toxic chemical reagents, as well as a minimal degree of complexity.
Efficient detection methods are likewise needed in order to detect nitro-containing compositions (e.g. nitrate and/or nitrite materials) on solid substrates. For example, the detection of nitrate and nitrite compounds ranging from fertilizers to explosive compositions is of current interest. The qualitative detection of nitro-containing compositions having explosive capabilities is especially important in connection with packaging materials, luggage, and related products. In recent years, a demand has developed for technology involving the detection of explosive compositions in airports and other public areas vulnerable to terrorist activities. Considerable interest exists in the rapid detection of explosives in trace amounts on luggage, shipping crates, and packages wrapped with paper, cloth, and the like. Also, it is desirable in many cases to detect such materials in soil and on human skin. Most modern explosives have physical characteristics which cause the adhesion thereof to solid objects. As a result, trace amounts of explosives are often found on the exterior surfaces of articles containing the explosives and on individuals handling the explosives. Detection of such trace amounts can be used to identify explosive articles and terrorist activities. Also, the detection of explosive materials in this manner may be useful in environmental decontamination/remediation processes at military installations and the like.
Many types of nitro-containing compositions having explosive capabilities are currently in use. For example, the following exemplary explosive materials are of particular concern: ##STR1##
Other explosive materials of interest include nitroglycerin, watergels, nitrocellulose, and potassium nitrate. It should be noted that the present invention shall not be limited to the detection of any particular nitrate, nitrite, and nitro-containing compositions, and shall likewise not be restricted to detection of the foregoing explosives which are provided for example purposes. Regarding the detection of explosives and other nitro-containing compositions, numerous methods have been investigated. For example, thermal neutron analysis has been used in which thermal neutrons react with nitrogen in the test materials (e.g. nitro-type explosives) to generate gamma rays as described in U.S. Pat. No. 4,851,687. X-ray detection methods have also been used in connection with nitro-containing compositions. In particular, dual energy systems based on established theories involving Compton scattering may be used for detection purposes as generally discussed in Fainberg, A., "Explosives for Aviation Security", Science, 255:1531-1537 (Mar. 20, 1992).
Regarding the detection of nitrate/nitrite materials and other nitro-containing compositions, a number of additional techniques have been employed including (1) electron capture analysis; (2) gas chromatography; (3) ion mobility spectrometry; and (4) ultraviolet spectrometry. Further techniques involving the detection of nitro-containing compositions are discussed in Bongiovanni, R., et al., "Analysis of Trace Amounts of Six Selected Poly-Nitro Compounds in Soils", Am. Ind. Hyq. Assoc. J., 45(4):222-226 (1984), as well as in U.S. Pat. Nos. 5,124,554; 5,114,662; 5,080,856; 5,006,299; 4,987,767; 4,882,121; 4,851,687; 4,788,039; 4,252,537; and 3,410,663.
Notwithstanding the foregoing analytical systems, a need remains for an effective method designed to detect the presence of nitro-containing compositions (including explosive materials) on solid substrates which is rapid, efficient, sensitive, and avoids the use of elaborate processing techniques. Furthermore, a need exists for such a method which is effective, yet portable and suitable for use in public places. The present invention satisfies this need by providing a highly efficient analytical process for detecting trace amounts of nitro-containing compositions (e.g. explosives and various nitrate/nitrite materials) on solid materials including but not limited to soil, packaging components, shipping crates, and containers.